Beamforming which is a kind of multiple antenna technology has been already adopted in early Long Term Evolution (LTE) standard release to enhance the coverage throughput. With respect to this technology, people were mainly focusing on azimuth domain so far. For example, how to form a horizontal beam using a certain weighting vector has been studied. In elevation domain, a fixed downtilt instead of a certain dynamic beam is supported in current LTE system.
With the increase of requirement on the elevation domain beamforming, 3D beamforming seems more and more important especially in urban area, in which users locate on different floors of the building. Using traditional horizontal beamforming technology can not serve these users very well, so the elevation domain and the horizontal domain both need to consider beamfoming, which is actually the 3D beamforming.
FIG. 1 shows an example of typical 3D beamforming. As shown in FIG. 1, the 3D beam sent from eNB (base station) 101 is serving the users on a certain floor. The beam could also serve the users on another floor dynamically. Therefore, the 3D beamforming could utilize vertical antenna units (or vertical beamforming) to further improve the system performance and potentially reduce the interference to other cells. To realize the 3D beamforming, the active antenna system (AAS) is the basis.
FIG. 2 shows a general AAS radio architecture in 3GPP TR 37.840. As shown in FIG. 2, a transceiver unit array (TUA) 201 assumes one-by-one mapping between the transceiver units #1, #2, . . . #K and the antenna ports. A radio distribution network (RDN) 202 could realize the mapping between the TUA 201 and the antenna array 203. By using the AAS system, a network could dynamically adjust all the elevation (or downtilt) and azimuth of the beam, and relevant beamwidth.
As the 3GPP TR 37.840 indicates, there could be different AAS deployment scenarios, such as Wide Area AAS (Macro AAS), Medium Range AAS (Micro AAS), and Local Area coverage AAS (Pico AAS), depending on the level of minimum coupling loss, the location of eNB (base station) antennas, etc. The range of each AAS scenario could be benefited from the 3D beamforming.
In 3GPP Release12, potentially two study items related with 3D beamforming would be discussed: one is the elevation beamforming and another is the FD-MIMO. The former assumes maximum 8 antenna ports and the latter could support {16, 32, 64} or even larger antenna ports. The antenna port is kind of logical signals which may be transmitted by several antenna units (physical antennas).
FIG. 3 shows a FD-MIMO with 8×8 antenna array structure. As shown in FIG. 3, the FD-MIMO which supports 64 antenna units potentially may need 64 CSI-RS ports to estimate the full dimension channels. In the 8×8 antenna array structure, the space of respective antennas may be 0.5λ.
FIG. 4 schematically shows CSI-RS regions per PRB (physical resource block) in release 11 of LTE. As shown in FIG. 4, the regions indicated with slash line “\” are the CSI-RS ports on the PRB for transmitting CSI-RS signals from the base station to user equipments. If it is true that 64 CSI-RS ports are needed for the FD-MIMO, the problem is that in current release 11 of the LTE, only 40 REs (resource element) are used as the CSI-RS ports per PRB. So how to allocate 64 CSI-RS ports to 40 REs on PRB is a problem.